ci: bump github actions to versions that natively target Node 24

Covers the "Node.js 20 is deprecated" warning emitted by actions/checkout,
actions/setup-node, actions/cache, actions/upload-artifact and
actions/download-artifact on their older releases.

Every bumped line carries an inline `# kilocode_change` marker since these
workflow files are shared with upstream OpenCode. When upstream bumps its
own action versions we just drop the markers on merge.

.github/workflows/check-opencode-annotations.yml

@@ -21,7 +21,7 @@ jobs:
     if: github.repository == 'Kilo-Org/kilocode'
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6 # kilocode_change
         with:
           ref: ${{ github.event.pull_request.head.sha || github.sha }}
           fetch-depth: 0